The vegetation dynamic around the Lake Arari – Marajo Island during the Late Holocene was studied. This research is based on pollen analysis, reflectance data, C/N relationship and AMS radiocarbon date of three sediment cores. The sampling sites are located in Santa Cruz do Arari, 6 meters height. A sediment core (65 cm) was sampled from the bottom of Lake Arari and the others (40 and 30 cm) were taken of herbaceous plain inside the lake drainage area. Probably, the herbaceous plain that dominates the eastern part of the Marajó Island began its development about 3800 cal years AP. Between 3800 and 3000 cal years A.P., the pollens diagram indicated a few mangrove pollens that suggests the final stage of an old expansion of a mangrove area. The C/N and _13C of organic matter accumulated during this period showed a marine influence. The low concentration of pollens between 3000 and 2000 cal years BP, probably due to change in flow energy, hinders the interpretation of the predominant vegetation type, but the C/N and _13C indicates a strong contribution of marine organic matter accumulated in studied sediments. This period of pollen absence may indicate an episode of difficult vegetation development due to, for example, the increase in marine influence. The last 2000 cal years A.P presented polinic content compatible with an herbaceous plain with a contribution trend of continental organic matter. The last 5 centimeters of the pollen diagram presented a predominance of Poaceae and Cyperaceae families which reflect the current predominant vegetation in the study area. The pollen diagram (PH1 and PH2) of the herbaceous plain, which colonizes the drainage basin of Lake Arari, describes a smaller spatial representation of vegetation, since they demonstrate only a constant predominance of Cyperaceae and Poaceae pollen that represents the main families of local sampling. The low depth of these profiles allowed to register the presence of herbs vegetation only from 460 and 590 cal years BP, respectively. Alternation between typical vegetation of freshwater and brackish water probably indicate changes in the porewater salinity that must be related to changes in freshwater discharge of rivers in the region and/or changes in sea level.
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